If a driver steps on an accelerator pedal to accelerate a vehicle when the vehicle is run on a frozen, snow-packed, or other slippery road surface, drive wheels may rapidly increase their speed and spin. If, on the other hand, the driver steps on a brake pedal to decelerate the vehicle, the drive wheels may rapidly decrease their speeds and lock up. These phenomena are hereinafter referred to as slippage. If such drive wheel slippage occurs, the behavior of the vehicle becomes unstable. In addition, steering control does not work to the detriment of running stability. It is therefore important to suppress such drive wheel slippage.
A conventional method of suppressing vehicle drive wheel slippage (slippage suppression control) is exercised in the event of drive wheel slippage to suppress the slippage by reducing the torque of an electric motor that drives the drive wheels. When the slippage is stopped, this method restores the reduced torque of the electric motor. A vehicle that uses such a slippage suppression method is described, for instance, in JP-A-1990-299402.